Electrolytic chlorinators and other kinds of electrochemical cells for the purification of waters have long been known. Many of these cells electrolyze concentrated brines and proportion chlorine or hypochlorites into the water circulation system. One such device is the "Aerion" Salt Chlorine Converter manufactured by the Franklin-Parker Corporation of Palo Alto, California. Other devices are described in British Pat. No. 1,323,713 and U.S. Pat. No. 3,351,542 and older patents. Other cells are made to electrolyze dilute salt solutions but these have required stepdown and variable transformers with fluid bypasses and/or temperature sensors and sometimes with reversal of currents to remove accumulated precipitates. Such devices are described in U.S. Pat. Nos. 3,458,414, 3,481,857, 3,223,242, and others. A cylindrical cell operating at 20 volts is offered for sale by the K-Ionics Company in Chatworth, Calif. Still others, such as U.S. Pat. No. 3,192,146, have used silver electrodes to utilize what is called "oligodynamy." All of these known cells suffer from inconveniences to those who use them and are prohibitively more expensive than alternative methods for simply adding chlorine or hypochlorite solutions at rates usually demanded by health authorities. Presently available cells tend to be undersized because of their expense. All of them require transformers, rheostats, or other means of controlling the voltage and current. Consequently, pool owners, those who make pools and those who sell supplies to pool owners are keenly interested in obtaining practical and low cost devices for the in situ chlorination of swimming pool waters.
The need for such devices is attested to by the large number of past unsuccessful attempts to invent a practical device for swimming pools. Actual chlorine requirements vary over wide ranges from about 0.1 to 10 pounds of chlorine per million pounds of circulating water which turns over a complete pools' capacity in 6 to 12 hours. The exact requirements depend on local conditions, as interpreted by local health authorities. The technical difficulties to satisfy the maximum requirements with equipment that frequently faces only the minimum requirements have not heretofore been solved in a practical way. The present use of large currents at low voltages requires special wiring, with expensive transformers, switches, rectifiers and controls for temperature, voltages, and fluid flows. Small currents at higher voltages have not been practical because a number of cells are then required to be in electrical series while the water is then made to pass in parallel or series through the cells. A number of electrical and plumbing inefficiencies are encountered with the parallel flows of water. When attempts are made to decrease the volts per cell by placing electrodes close together, hard water impurities tend to precipitate quickly in some of the electrolysis zones thereby plugging the cells and stopping the electrical current and flow of water. If voltage decreases per cell are sought by increasing salt contents in the waters, too much salt becomes objectionable to the swimmers. On the other hand, too little salt leads to the requirement of very large electrodes at small current densities thereby making the cells prohibitively expensive to manufacture and to maintain in an operating condition. Also, no one has solved the electrical distribution problems encountered with the introduction of substantially full line 120 or 240 volts into bipolar electrolysis cells in proximity to metal equipment and swimmers in the same electrolyte.
In other attempts to meet the extreme demands for purification of pool waters by electrolysis of dilute solutions, cells have been designed with extensive paths for the prolonged electrolysis of the solutions passing through. U.S. Pat. No. 3,305,472 describes one such cell for use with bromide additions with an example which shows the same kind of electrolysis will not work with dilute chloride solutions. This approach of extensive electrolysis of bromide solutions led to the production of bromates and perbromates which are of no use for purifying swimming pools and, of course, the production of chlorates or perchlorates is also undesirable.
One of the problems encountered with the introduction of high voltages directly into a set of bipolar electrodes is the creation of voltage gradients between metallic parts or swimmers and certain of the electrodes, even if those metallic parts or swimmers are properly grounded. This existing grounding problem may be understood by reference to FIG. 1 wherein an ordinary alternating current and voltage source is shown at 10 with one side grounded at 17. The nature of commercial, 60 cycle, alternating current in the United States is such that the ungrounded, "hot," line alternates from a nominal 120 volts positive with respect to ground to a nominal 120 volts negative with respect to ground and with a wave form substantially like a sine wave wherein the root-mean-square, rms, ac voltage or current is only 0.706 times the peak voltage or current. Thus, the 120 rms volt source in FIG. 1 may have a peak voltage of about 170 volts ac. When this large peak voltage is fed into the bipolar cell 16, it means that grounded persons or metal parts are as much as 170 peak volts separated from that ungrounded lead. Whether a grounded person is 170 volts more positive or 170 volts more negative than this "hot" lead will generally make no difference if he feels a shock. However, when pool walls, pump parts, valves or any other metallic components are in contact with the same water which also contacts the "hot" electrode, the polarity with respect to ground can make much difference. Thus, when the "hot" electrode is positive with respect to ground, all other metal parts in the system are negative with respect to the "hot" electrode and will merely tend towards evolution of hydrogen gas and with no harm to that part. When the "hot" electrode is negative with respect to ground, however, all grounded metal parts in the system are positive with respect to the "hot" electrode and tend to be electrochemically oxidized or corroded. Even a very small amount of such corrosion over a continuous period of time could lead to catastrophic damage of metal parts in the pool system. This grounding problem is numerically the same when the primary power source is a nominal 240 volt ac line, for then a third wire may be grounded but the two "hot" lines each alternate oppositely by 120 rms volts positive and negative with respect to ground. Thus, at 240 rms volts, no electrode is more than 120 rms volts from ground, just as desribed above for a 120 rms volt system. With center-tapped isolation transformers, between the power source 10 and rectifier 15, the grounding problem can be cut in half for the 120 rms power source, but even this partial aid is inadequate for safe grounding and will raise the cost of the device and its circuitry to price levels which tend to make the device impractical for the owners of swimming pools.
As a result of these fundamental problems, chlorine or bromine chemicals are generally added directly, either as concentrated solutions or as slowly dissolving sticks or pellets. These halogen concentrates are expensive, corrosive, poisonous, and require a substantial amount of experience and expertise to use properly. Some pool owners find the presently available chlorination methods so complicated (there is too much or too little chlorine) that they dump in large excesses of chlorine chemicals during the night counting on sunlight to "burn off" the excess during early morning hours. This is a dangerous and inefficient use of chlorine. Electrochemical devices on the market are not flexible enough to satisfy the extreme requirements of local health authorities and, therefore, require a combination of electrolysis with chemical additions.